In a cellular wireless data communication system, a base station is associated with a sector. Mobile stations within the sector may access the Internet or other data networks via the base station. Typically, each mobile station only communicates with a single base station (strictly speaking, the mobile stations may be in handoff with several base stations, however such details are outside the scope of this disclosure and do not prevent the applicability of the invention). The base station, however, must be able to communicate with all of the mobile stations within its sector.
The connection between a base station and a mobile station comprises a “forward link” (FL) which refers to the flow of information from the base station to the mobile station, and a “reverse link” (RL) which refers to the flow of information from the mobile station to the base station. The information transmitted on the forward and reverse links mostly consists of user data (such as web pages, electronic mail, etc.) encoded and modulated appropriately, but it also includes control information (such as power control, data rate control, etc.)
The terms “capacity” or “(overall) throughput” from a base station refers to the amount of user data bits transmitted per second (measured in units of bits per second) on the forward link to all mobile stations in the sector. A similar definition also holds for the reverse link capacity or throughput.
Contrary to a cellular wireless voice communications system which requires equal amounts of throughput on the forward and reverse links, a cellular wireless data communication system usually requires higher throughput on the forward link. This is because a typical Internet access usually involves “downloading” web pages or computer files from the Internet to the user's mobile station (such as a laptop equipped with proper hardware), although “uploading” large files (such as large electronic mail attachments) are also very common. The fact that the Internet is an extremely large source of information is the main reason for asymmetrical throughput requirements on the forward and reverse links.
Another important difference between a voice communications system and a data communications system is that in data communications, the delay in receiving data can be tolerated to a much larger degree than the delay in voice communications. With this flexibility, a data-only communications system may provide much better throughput than a system that transmits data over a voice-optimized network.
One such cellular wireless data communications system is the cdma2000 1×EV-DO system (also known as IS-856), an evolution of IS-95 family of CDMA standards, designed in particular for wireless Internet access. The name “1×EV-DO” refers to the relation to the cdma2000 family (“1×”) and the evolution of the standard (“EV”) for data optimized (“DO”) operation. The 1×EV-DO system is optimized for wireless Internet access, for which a high data throughput on the Forward Link is desirable. Furthermore, the Forward Link data transmissions are not as delay-sensitive, as compared to a voice transmission system. As a result, the base station can transmit data to individual mobile stations discontinuously in a time-division multiplexed manner. The 1×EV-DO system provides a 1.25 MHz channel, and utilizes the same RF components as used in the existing IS-95 networks and devices. 1×EV-DO has the same coverage as IS-95 and cdma2000 networks.
The 1×EV-DO system defines 12 different data rates (and the corresponding data packet structures, such as packet duration, modulation type, etc) on the forward link ranging from 38.4 kbps to 2.4 Mbps (in addition to the null rate). Although the invention will be mostly described in the context of a 1×EV-DO system in this disclosure, it may also be applied to certain other systems.
In the 1×EV-DO system, the data rate at which a mobile station is served is determined by the mobile station, based on the quality of the received signal from the base station. The quality of the received signal is typically measured in terms of the signal-to-interference-and-noise ratio (SINR). As the SINR level of transmissions received by a mobile station increases, the mobile station becomes capable of successfully receiving data at higher data rates. The mobile station predicts the maximum data rate that it can reliably receive based on its measurements and predictions of the SINR conditions, and requests this maximum data rate from the base station via the reverse link data rate control (DRC) channel. The SINR, however, typically fluctuates in time over a wide range. This is mainly due to the movement of the mobile station and the surrounding objects. Consequently, the maximum data rate that a mobile station can reliably support is dynamically determined based on the channel conditions. For this purpose, the 1×EV-DO system allows the requested data rate to be updated as often as 600 times per second.
In the 1×EV-DO system, the mobile stations are served in a time-division multiplexed manner. Unlike a classical time-division multiple access (TDMA) system, however, the time intervals over which a mobile station is served are not pre-allocated. Rather, the base station dynamically makes scheduling decisions to determine which mobile station to serve at a given time based on the periodically received DRC information from all mobile stations.
The base station may accomplish scheduling decisions for data transmissions to the mobile stations in its sector by utilizing a scheduling algorithm. One type of scheduling algorithm is referred to as a “proportional fair scheduler”. A proportional fair scheduler is designed to balance the often conflicting requirements of fairness of service among the mobile stations against the maximization of the overall forward link sector throughput. More specifically, the proportional fair scheduler performs scheduling decisions by selecting the mobile station for which a certain decision metric is maximized. The decision metric for a given mobile station is the ratio of the requested data rate by that mobile station to a weighted average of the data throughput transmitted so far by the base station to that mobile station.
The effect of using the proportional fair scheduler is to schedule mobile stations when their SINR values are large with respect to their average values, thus increasing the overall throughput of the base station while achieving fairness among the mobile stations. As a result of the SINR fluctuations seen independently by each mobile station, the scheduler tends to distribute service among the mobile stations, each at or close to its peak supportable data rate.
With the use of the proportional fair scheduler, the overall throughput of the base station increases in general with the increasing number of mobile stations in the sector. This is because of the fact that, at any given time, the probability of finding a mobile station that requests a very high data rate relative to its own average throughput increases as the number of mobile stations in the sector increases. The gain in the overall base station throughput as a function of the number of mobile stations is referred to as the “multi-user diversity gain”. Multi-user diversity gain generally increases when the number of mobile stations in the sector increases, and also when the dynamic range of the SINR fluctuations seen by each of the mobile stations increases (even when the average SINR may remain the same).
While it may not be practical to increase the number of mobile stations in the sector, there are ways to artificially increase the dynamic range of the SINR fluctuations seen by the mobile stations. One way to accomplish this is to use a forward power modulation (FPM) scheme that induces SINR fluctuations. In one such scheme, a two-element antenna array is utilized to form a highly directional antenna gain pattern (also referred to as a “beam”). The two antennas are placed sufficiently close to each other and their boresights are aligned. The power gain of the second antenna is kept at a certain fixed fraction of the first antenna. The phase of the second antenna relative to the first antenna is, however, varied linearly in time at a fixed rate, such as 2 cycles per second. The result is a time-varying antenna gain pattern where the highly directional beam sweeps across the sector periodically.
In another embodiment, an antenna system by which a FPM style approach can be developed may comprise an array of highly directional antenna elements, each directed towards a fixed sub-sector of a given sector and forming a beam sub-pattern. The antenna gain sub-patterns are non-overlapping to a large degree, alleviating the need to control relative phases of the array elements. The power gains of each element, however, can be individually controlled with the only restriction being that the total transmitted power from the antenna array is maintained at a fixed value and the power transmitted from individual array elements are maintained between a lower and an upper level to avoid excessive interference to other sectors. In such a system, an FPM method would be based on periodically changing each element (such as a few hundred milliseconds in every second) of the array and setting its power gain at a high value while setting the remaining element power gains to some predetermined levels. Such a scheme would also result in a highly directional overall antenna gain pattern periodically sweeping across the sector. In such a case, the index of the antenna to which the highest power gain is allocated would be the controlling parameter (as opposed to the previously described two antenna case in which the phase of the second antenna was the controlling parameter).
In all of the above described FPM schemes, the highly directional antenna gain pattern periodically sweeps across the sector. As the beam sweeps over a particular mobile station, the SINR value for that mobile station tends to peak and then fall off. Similar approaches to artificially induce SINR fluctuations include opportunistic beam forming (OBF), and various schemes based on other antenna structures.
In this disclosure, we will often use the term “rotating a beam” to refer to varying the parameters of a directional antenna gain pattern in time so as to vary the antenna gain pattern and provide a high signal strength to various regions of the sector in a time-varying (periodic or aperiodic) manner. The term is used for conciseness, and does not necessarily imply an actual rotation of a fixed shaped beam at a fixed angular velocity. In one embodiment, the directional antenna gain pattern may consist of multiple local peaks that sweep a sector in time.
It should be noted that another advantage of FPC, OBF, and similar methods of artificially inducing SINR fluctuations is that especially if properly coordinated among all base stations, these schemes may also improve average SINR received by a mobile station (especially one that is in handoff between two or more sectors) by reducing the interference received from other sectors while the desired base station is beam forming to the mobile station.
The FPM scheme and similar schemes work well if there are a large number of mobile stations distributed throughout the sector, because it becomes more probable, as the number of mobile stations increase, that at least one of them will be receiving a high SINR level at any given time and will therefore be selected by the scheduler. If, however, there are only a few mobile stations in the sector, the FPM scheme and similar approaches may not perform well. This is because there are periods of time when the beam is not directed to any of the mobile stations, so they all have relatively low SINR levels and therefore request low data rates from the base station. Thus, no multi-user diversity gain is achieved. In fact, if the number of mobile stations is low enough, the FPM scheme and similar schemes may actually cause a degradation in the overall data throughput from the base station.